It is known, in a satellite communications system, for a user terminal, perhaps in the form of a radio telephone handset, not dissimilar to a cellular radio telephone handset, to communicate with an earth station, via a satellite, to establish a telephone call or to receive a telephone call, by means of the earth station linking into the terrestrial wire and cable system or into another radio telephone system.
There are twin purposes relating to establishing the exact position of the user terminal on the surface of the earth. Firstly, in order to know how to direct a radio signal to or from a user terminal, when required, from a particular satellite at a particular time, it is necessary to know the approximate location of the user terminal so that the appropriate beam from the appropriate satellite can be selected to cover the portion of the Earth's surface where the user terminal is located. Secondly, in a satellite communication system, in order that call barring, local billing or other restrictions based on the territory wherein the user terminal may be operated can be observed, it is necessary to determine the location of the user terminal with sufficient accuracy for the necessary restrictions to be imposed.
It is known to provide a user terminal where the individual user terminal employs "Global Positioning by Satellite" (GPS) to determine, with some great accuracy, the position of the user terminal on the surface of the earth. The user terminal then transmits, to the earth station, via the satellite or satellites involved in communications, its exact position which is then used by the earth station, in subsequent interactions with the user terminal, to control the fiscal and mechanical aspects of the communication activity with the user terminal. An example of such a system is to be found in European Patent EP 0562 374 by Motorola Corporation filed Mar. 27, 1993. The GPS system tends to be very slow of access, requires a very sophisticated receiver of a costly nature, and the GPS satellites can often be totally inaccessible to the user terminal, in sufficient simultaneous numbers for a position determination to be achieved. In addition, the accuracy of the position determination is well in excess of what is actually required for satellite communications purposes.
Such systems require unnecessary complication of the handset, in order that the handset may be capable both of communications and of GPS measurements.
It is advantageous, therefore, to provide a system and method whereby the position, on the surface of the earth, of the user terminal or handset can be determined with sufficient accuracy for communication and fiscal purposes without undue complication of the handset or user terminal.
In another system, it is merely necessary for the satellite to receive a transmission from the user terminal, in response to the earth station, via the satellite, requesting radio communication from the user terminal, for the earth station to use the delay in response from the user terminal for ranging purposes and to use the doppler shift on the received frequency from the user terminal, together with a foreknowledge of the position and velocity of the satellite, to determine, with a certain degree of precision, the position of the user terminal on the surface of the earth.
Unfortunately, if the cost of the user terminal is to be kept at realistic limits, consistent with the trade in handheld mobile radio telephones, the accuracy of the crystal clock or other frequency source within the user terminal cannot be made consistent with sufficient accuracy of determination of the position of the user terminal, on the surface of the earth, for the functional and fiscal aspects connected with operation of a satellite telephone communication system.
Even when these problems are overcome, there is always an uncertainty as to which of a possible plurality of positions the user terminal or handset actually occupies. If the possible positions are widely separated, so as to fall into separate radio beams from the satellite, it is always possible to resolve the ambiguity by detecting in which beam the user terminal lies. When, however, the beams are very large, or the possible points very close together, so that two or more possible points fall within a single beam, resolution of the uncertainty becomes a problem. The present invention seeks to provide a system and method to address this problem.